mine went off then started working again and aint gone off since and its been 4 months now. thinks its all that banging about when your ragging it down a country lane. thats my view on it anyway. (loose connection)
I bet its the selector shaft seal (ive changed many on lupo's and polo's) the seal itself is no more than £2 but they may charge half hour to fit it, so i say no more the £40 is reasonable.